Cleaning solution and method for cleaning semiconductor substrates after polishing of copper film

ABSTRACT

A cleaning solution for cleaning a semiconductor substrate is formed by mixing an amount of citric acid and an amount of ammonia in deionized water. In one embodiment, the amount of citric acid is in a range from about 0.18% by weight to about 0.22% by weight and the amount of ammonia is in a range from about 0.0225% by weight to about 0.0275% by weight, and the cleaning solution has a pH of about 4. A method for cleaning a semiconductor substrate having a polished copper layer in which a concentrated cleaning solution is mixed with deionized water proximate to a scrubbing apparatus also is described.

[0001] This application is a continuation-in-part of U.S. patentapplication Ser. No. 09/227,494, entitled “Methods and Apparatus forCleaning Semiconductor Substrates After Polishing of Copper Film,” filedJan. 7, 1999, which is a continuation-in-part of U.S. patent applicationSer. No. 08/955,393, entitled “Methods and Apparatus for CleaningSemiconductor Substrates after Polishing of Copper Film,” filed Oct. 21,1997. The disclosures of these priority applications are incorporatedherein by reference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] This invention relates to methods and apparatus for processingand cleaning a substrate, and more specifically to methods and apparatusfor cleaning semiconductor substrates after polishing of copper films.

[0004] 2. Background Information

[0005] In the manufacture of advanced semiconductor devices, copper (Cu)is beginning to replace aluminum (Al) as the material for metallization.Cu has become desirable due to its lower resistivity and significantlyimproved electromigration lifetime, when compared to Al.

[0006] One process for Cu metallization uses a dual damascene approach.As illustrated in FIG. 1a, a dielectric layer 110 is deposited above asubstrate 100. Dielectric layer 110 may be made up of materials such assilicon dioxide. Vias and/or trenches 120 are then formed in thedielectric layer 110, as illustrated in FIG. 1b. Vias/trenches 120 maybe formed, for example, using dry etching techniques. Next, a thin layerof barrier material (barrier layer) 130, for example, tantalum (Ta),titanium (Ti), or titanium nitride (TiN) is deposited as illustrated inFIG. 1c. After barrier layer 130 is deposited the vias/trenches 120 arefilled with copper (Cu) layer 140, as illustrated in FIG. 1d. Cu layer140 may be deposited using well known deposition techniques, forexample, chemical vapor deposition (CVD), physical vapor deposition(PVD), or electroplating. In order to isolate the copper interconnects,as illustrated in FIG. 1e, the excess copper layer 140 and barrier layer130 must be removed.

[0007] One method for removing the excess copper layer 140 and barrierlayer 130 is polishing the surface of the substrate, for example,polishing using chemical mechanical polishing (CMP). In a CMP process,the semiconductor substrate is polished with a slurry containingabrasive particles, such as alumina particles, and an oxidant, such ashydrogen peroxide. In the CMP process, contaminants are introduced whichinclude particles and/or metal contamination on the copper layer 150,dielectric surface 160, and in the dielectric subsurface 165.

[0008] Regardless of how the CMP process is performed, the surface ofsemiconductor substrate must be cleaned of contaminants. If not removed,these contaminants may affect device performance characteristics and maycause device failure to occur at faster rates than usual. Cleaning thesemiconductor substrate after chemical mechanical polishing of coppermay be necessary to remove such contaminants from the copper layer anddielectric layers.

[0009] One method for cleaning the semiconductor substrate afterpolishing of the copper layer is brush scrubbing. Brush scrubbing,whether single-sided or double-sided brush scrubbing, is the industrystandard for cleaning oxide and tungsten CMP applications. However,there are several problems associated with applying brush scrubbing topost copper CMP cleaning.

[0010] One such problem is brush loading. During the CMP process, thetop surface of the copper layer may be oxidized and forms copper oxide,for example copper oxide (Cu₂O or CuO) or copper hydroxide (Cu(OH)₂). Inbasic or neutral pH cleaning environments, the copper oxide or copperhydroxide does not dissolve and may be transferred to the brushes, thusloading the brushes. The contaminated (or loaded) brushes may thentransfer the copper oxide or copper hydroxide contaminants tosubsequently processed substrates during cleaning.

[0011] For tungsten and other oxide applications, brush loading could becurtailed by adding a dilute ammonium hydroxide (NH₄OH). In the presenceof NH₄OH, part of the copper oxide may form Cu(NH₃)²⁺ complex and may bedissolved; however, due to the high pH environment, the dilute ammoniumhydroxide has been found to be insufficient to prevent brush loading ofcopper oxide. Additionally, it has been found that scrubbing with diluteammonium hydroxide also causes etching of the copper layer and may causeserious surface roughening.

[0012] Brush loading may also occur when alumina particles are used inthe copper CMP process. In neutral or inorganic acid (e.g., HCl)cleaning environments, there is an electrostatic attraction betweenalumina particles and the silicon dioxide surface which makes itdifficult to remove the alumina particles from the surface of thedielectric material. Because of the electrostatic attractive force, thealumina particles may also adhere to the brush and cause another brushloading problem with similar effects to those discussed above.

[0013] Yet another problem caused by the CMP process is that the surfaceand subsurface of the dielectric layer may become contaminated duringpolishing with metal from the copper layer and barrier layer as well asother contaminants from the slurry. During the CMP process,contaminants, especially metal contaminants, may penetrate into thedielectric layer up to approximately 100 angstroms (Å) from the surface.Again, these contaminants may affect device performance characteristicsand may cause device failure.

SUMMARY OF THE INVENTION

[0014] In one aspect of the invention, a cleaning solution for cleaninga semiconductor substrate is provided. The cleaning solution may beformed by mixing an amount of citric acid and an amount of ammonia indeionized water. In one embodiment, the amount of citric acid is in arange from about 0.18% by weight to about 0.22% by weight and the amountof ammonia is in a range from about 0.0225% by weight to about 0.0275%by weight, and the cleaning solution has a pH of about 4.

[0015] In another aspect of the invention, a method for cleaning asemiconductor substrate having a polished copper layer is provided. Inthis method, a concentrated cleaning solution is provided. Thesemiconductor substrate having the polished copper layer is placed in ascrubbing apparatus. In one embodiment, one unit volume of theconcentrated cleaning solution is mixed with 20 unit volumes ofdeionized water proximate to the scrubbing apparatus to obtain acleaning solution. The semiconductor substrate is scrubbed in thescrubbing apparatus in the presence of the cleaning solution.

[0016] Additional features and benefits of the present invention willbecome apparent from the detailed description, figures, and claims setforth below.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017] The present invention will be understood more fully from thedetailed description given below and from the accompanying drawings ofvarious embodiments of the invention, which, however, should not betaken to limit the invention to the specific embodiments, but are forexplanation and understanding only.

[0018]FIG. 1a illustrates a semiconductor substrate having a dielectriclayer deposited thereon.

[0019]FIG. 1b illustrates the semiconductor substrate of FIG. 1a aftervias and/or trenches are formed in the dielectric layer.

[0020]FIG. 1c illustrates the semiconductor substrate of FIG. 1b after athin barrier layer has been deposited thereon.

[0021]FIG. 1d illustrates the semiconductor substrate of FIG. 1c after alayer of Copper material has been deposited thereon.

[0022]FIG. 1e illustrates the semiconductor substrate of FIG. 1d afterchemical mechanical polishing of the excess copper layer and barrierlayer.

[0023]FIG. 2 illustrates one embodiment of a scrubber system.

[0024]FIG. 3 illustrates a flowchart of one embodiment of the process ofthe present invention.

DETAILED DESCRIPTION

[0025] A cleaning solution and a method for cleaning semiconductorsubstrates after polishing of copper film are disclosed. In thefollowing description, numerous specific details are set forth such asspecific materials, processes, parameters, dimensions, etc. in order toprovide a thorough understanding of the present invention. It will beapparent, however, to one skilled in the art that these specific detailsneed not be employed to practice the present invention. In otherinstances, well known materials or methods have not been described indetail in order to avoid obscuring the present invention.

[0026] The following description describes cleaning solutions, methods,and apparatuses for cleaning a semiconductor substrate. In oneembodiment, the cleaning of a semiconductor wafer occurs after theformation of copper interconnect(s) and chemical mechanical polishing(CMP)/planarization of that copper interconnect(s). Processes forformation of copper interconnects in semiconductor device fabricationare well known in the art and are therefore not described in detailherein.

[0027] It should also be noted that the term “semiconductor substrate”used herein refers to a silicon semiconductor substrate or a partthereof, such as gallium arsenide, upon which device layers have been orare going to be formed. It should also be noted that the term substrateincludes but is not limited to fully processed, semi-processed, orunprocessed substrates with semiconductor materials thereon.

[0028] Additionally, although the cleaning solutions, methods andapparatuses for cleaning are described in conjunction with the scrubbingof a semiconductor substrate or wafer, it will be appreciated that anysimilarly shaped, i.e. generally flat substrate, may be processed by themethods and apparatuses of the present invention. Further, it will beappreciated that reference to a semiconductor substrate or wafer mayinclude a bare or pure semiconductor substrate, with or without doping,a semiconductor substrate with epitaxial layers, a semiconductorsubstrate incorporating one or more device layers at any stage ofprocessing, other types of substrates incorporating one or moresemiconductor layers such as substrates having semiconductor oninsulator (SIO) devices, or substrates for processing other apparatusesand devices such as flat panel displays, multichip modules, etc.

[0029] In one embodiment, a cleaning solution used to cleansemiconductors substrates is made up of deionized water, an organiccompound, and an inorganic compound, all of which are combined, creatingan acidic pH environment for cleaning the surface of a semiconductorsubstrate. Again, such cleaning may occur after polishing a copperlayer. The use of an acidic pH environment helps dissolve copper oxideand alleviates some of the problems of brush loading discussed in thebackground of the invention. It is advantageous to keep the acidic pHenvironment within a pH level range of approximately 1-6. In oneembodiment, the acidic pH environment has a pH level in the range ofapproximately 2-4.

[0030] The use of an organic compound (e.g., an organic acid) helps toform metallic complex compounds, thereby helping removal of the metalcontamination from the surface of the dielectric layer and from thesurface of the brush. Some examples of organic acids that may be usedinclude: citric acid, malic acid, malonic acid, succinic acid, or anycombination of such organic acids.

[0031] In one embodiment, the organic compound is dissolved in deionizedwater (DIW) in a concentration range of approximately 100 ppm to 2% byweight. In alternate embodiments, a more preferred concentration rangemay be approximately 200 ppm to 0.1% by weight. In one embodiment, whencitric acid is the organic compound, concentration of the citric aciddissolved in deionized water is approximately 0.2% by weight.

[0032] The use of an inorganic compound helps to change theelectrostatic forces between the particles and surfaces of the brush andsubstrate in order to make them repulsive. Thus, the particles repel thebrushes and the substrate and the substrate and brushes repel theparticles, providing favorable conditions for particle removal. In oneembodiment, the inorganic compound in the cleaning solution may beammonium hydroxide (NH₄OH), the ammonium salt of an inorganic acid(e.g., ammonium chloride (NH₄CL), ammonium fluoride (NH₄F)), or ananionic surfactant.

[0033] It is desirable to dissolve the inorganic compound in deionizedwater (DIW) in a concentration range of approximately 100 ppm to 2% byweight. In one embodiment, where the inorganic compound is an ammoniumcompound, the ammonium compound is dissolved in DIW such that theconcentration range is approximately 200 ppm to 0.1% by weight. In oneembodiment, the concentration of ammonium hydroxide (when used in thecleaning solution) when dissolved in DIW is approximately 0.02% byweight.

[0034] One example of the many different ways to formulate the cleaningsolution is: 0.02% NH₄OH, 0.2% Citric Acid by weight mixed in DIW. ThepH level of the solution in this example is approximately 4.

[0035] Another exemplary way to formulate the cleaning solution is bymixing citric add in a concentration range of approximately 0.18%-0.22%by weight and ammonia in a concentration range of approximately0.0225%-0.0275% by weight in DIW. The resultant cleaning solution has apH of about 4. In one embodiment, the cleaning solution is formed bymixing about 0.2% by weight of citric acid and about 0.025% by weight ofammonia in DIW.

[0036] The ammonia component may be derived by diluting commerciallyavailable ammonia, which typically contains about 37 grams of ammoniadissolved in 100 grams of DIW, to obtain the desired concentration ofammonia. It will be apparent to those skilled in the art that ammoniaforms ammonium hydroxide when mixed in DIW.

[0037] Those skilled in the art will appreciate that numerous chemicalreactions will take place between the starting materials when they aremixed in DIW. For example, in the case of a cleaning solution formed bymixing about 0.2% by weight of citric acid with about 0.025% by weightof ammonia in DIW, the thus-formed solution will contain at least thefollowing species: ammonium hydroxide, ammonium ions, citrate ions, andcitric acid. The relative percentages of these species for thisexemplary cleaning solution have been calculated to be as follows(assuming room temperature (25° C.): about 0.025% by weight of ammonium(NH₄ ⁺) ions, about 3 ppb of ammonium hydroxide (NH₄OH), about 0.175% byweight of citrate ions, and about 0.025% by weight of citric add. Thoseskilled in the art will appreciate that the relative amounts of thesespecies may vary at temperatures other than room temperature.

[0038] For ease of transportation from the place of manufacture to theplace of use, the above formulations of the cleaning solution may beconcentrated and diluted later to the proper concentration by the enduser at the site of use, e.g., proximate to a scrubbing apparatus.Moreover, diluting the concentrated cleaning solution with DIW reducesthe frequency with which the container from which the cleaning solutionis dispensed must be replaced in a semiconductor fabrication facility.For example, one exemplary concentrated cleaning solution may beproduced by mixing citric acid in a concentration range of approximately3.6%-4.4% by weight and ammonia in a concentration range ofapproximately 0.45%-0.55% by weight in DIW. The resultant concentratedcleaning solution has a pH of about 3.8. In one embodiment, theconcentrated cleaning solution is formed by mixing about 4% by weight ofcitric acid with about 0.5% by weight of ammonia in DIW. In oneembodiment, the concentrated cleaning solution is diluted by mixing oneunit volume of the concentrated cleaning solution with 20 unit volumesof DIW.

[0039] If desired, other suitable organic acids may be substituted forcitric acid in the cleaning solution formulations described above. Byway of example, suitable organic acids include malic acid, malonic acid,succinic acid, oxalic acid, or any mixture thereof. It should also beknown that other suitable sources of ammonia besides ammonium hydroxidemay be used to create the cleaning solution such as, for example,ammonium chloride and ammonium fluoride.

[0040] If an ammonium salt, such as for example, ammonium chloride orammonium fluoride, is used, the concentration range of the ammonium saltdissolved in DIW may be approximately 0.05%-0.1% by weight. Also, if ananionic surfactant is used, the concentration range of the anionicsurfactant dissolved in DIW may be approximately 50 ppm to 0.2% byweight.

[0041] In one embodiment, a cleaning solution of DIW, ammonium salt anda chloride compound is used to clean the semiconductor substrates. Inone embodiment, the pH level of the solution is in the range ofapproximately 2-4. The ammonium salt may be one of the ammonium saltsdiscussed above. In one embodiment, it is desirable to dissolve theammonium salt in deionized water (DIW) in a concentration range ofapproximately 200 ppm to 0.2% by weight. In one embodiment, the ammoniumsalt is dissolved in the DIW in a concentration of approximately 0.1% byweight. The chloride compound may comprise hydrochloric acid (HCL),ammonium chloride, or a combination of the two. In one embodiment, it isdesirable to dissolve the chloride compound in DIW in a concentrationrange of approximately 0.1% to 1% by weight. In one embodiment, thechloride compound is dissolved in DIW in a range of approximately 0.1%by weight.

[0042] The cleaning solution may comprise a mixture of chemicals in DIWcontaining an organic acid, ammonium salt of an inorganic acid, or ananionic surfactant in an acidic pH environment. In such a case, theorganic acid may be one of those organic adds listed above, and with aconcentration, when dissolved in DIW, of approximately 0.2% by weight orin a concentration range of 0.1% to 1% by weight. In the case of usingan anionic surfactant, it is desirable to dissolve the anionicsurfactant in DIW in a concentration range of approximately 50 ppm to0.2% by weight and approximately 0.2% by weight in one embodiment.

[0043] In one embodiment, the chemicals of the present invention arepre-mixed in the same cleaning solution to simultaneously solve severalproblems related to post copper CMP cleaning using a brush scrubber.Cross contamination from substrate to substrate and within the samesubstrate are therefore reduced substantially, or even prevented in thissimple approach. Hydrochloric acid (HCl) may also be added to thesolution to adjust pH and help dissolve copper oxide.

[0044] The present invention covers various formulations of the cleaningsolution, and that each component in the solution may be replaced bydifferent chemical that has similar properties. As described in thebackground of the invention, after the copper interconnects on asemiconductor substrate have been planarized using CMP techniques, it isnecessary to clean the semiconductor substrate and remove anycontaminants from the surface and subsurface of the semiconductorsubstrate. One such technique for removing contaminants from thesemiconductor substrate is scrubbing the semiconductor substrate(substrate).

[0045] As an example, and not by limitation, the present invention isdescribed in conjunction with a scrubbing process, more specifically, ascrubbing process in which both sides of the wafer are scrubbedsimultaneously. The scrubber may include a number of stations. Each ofthese stations represents one or more steps in the substrate cleaningprocess. Contaminated substrates are loaded at one end of the system andcleaned and dried substrates are unloaded from the other end of thesystem. Example of a systems of this type are the DSS-200™ Scrubber andthe Synergy™ Scrubber available from Lam Research Corporation ofFremont, Calif.

[0046]FIG. 2 represents a cross sectional view of a Synergy™configuration (cleaning system). Usually, the contaminated substratesare delivered to the cleaning system after chemical mechanicalplanarization (CMP) from a wet bench or from other processes resultingin contamination. At the start of the cleaning process contaminatedsubstrates are loaded into a wafer cassette 280 (cassette) and thecassette 280 is then placed into the wet send indexer station 210. Aftercassette 280 is placed into wet send indexer station 210, the substratesare automatically removed from the cassette 280 and placed, one at atime, into the outside brush station 220.

[0047] In the outside brush station 220, a substrate is processedthrough a first scrub. During the first scrub, the cleaning solution maybe applied to the substrate in several different ways. For example, inone embodiment the cleaning solution is sprayed onto the substrate. Inanother embodiment the cleaning solution is applied to the substratethrough brushes 221. Yet another embodiment applies the cleaningsolution by dripping the cleaning solution onto the substrate.

[0048] The scrubbed substrate is then automatically removed from theoutside brush station 220 and placed into the inside brush station 230.In the inside brush station 230, the substrate is processed through asecond scrub. In the inside brush station 230 the cleaning solution maybe applied to the substrate in a similar manner as in outside brushstation 220.

[0049] After the second scrub the substrate is then automaticallyremoved from the inside brush station 230 and placed into the rinse,spin and dry station 240. Rinse, spin, and dry station 240 rinses,spins, and dries the substrate. At this point the wafer has beencleaned.

[0050] Once the rinse, spin, and dry steps have been completed thesubstrate is then transported from the rinse, spin, and dry station 240to the output station 250 where the substrate will be placed intocassette 281. The transfer is usually carried out by a robotic arm whichlifts the substrate out of the rinse, spin, and dry station 240 by itsedges and places it into the cassette 281. The cassette is thentransferred to storage or to another cleaning or processing system.

[0051] It will be apparent to one of ordinary skill in the art that someof the steps in the cleaning system described above may occur in anotherorder and/or with various solutions depending upon the substrate orsubstrate layer being cleaned. For example, different cleaningsolutions, such as water, citric acid, ammonium hydroxide, ammoniumcitrate, and hydrofluoric acid solution (or mixtures of solutions) maybe used in one of the brush stations. Also, other systems may includeone brush station, or more than two brush stations. Moreover, othersystems may omit one or more of the above stations/steps and may includeadditional processing stations, such as a CMP station.

[0052] While the previous description illustrates a cleaning system inwhich both sides of the substrate are scrubbed simultaneously, thetechniques described herein may be used in other cleaning systems andprocesses. For example, a cleaning system in which only a single side ofthe substrate is scrubbed or a cleaning system in which the substrate iscleaned with chemical spray.

[0053]FIG. 3 illustrates one embodiment of a cleaning process. At step310, the copper layer is planarized using chemical mechanical polishing.It should be noted that other techniques for planarization of the copperlayer may be used and that it may still be desirable to clean thesemiconductor substrate using the present invention after suchplanarization in order to remove potential contaminants from thesubstrate surface and/or subsurface.

[0054] At step 320, the polished semiconductor substrate is then placedin a scrubber. The substrate is then scrubbed, at step 330, to removethe contaminants caused by the polishing process. During scrubbing, acleaning solution, such as described above, is applied to the substratein order to aid and/or effectuate the removal of the contaminants (step340). This cleaning solution may be used in either outside brush station220 or inside brush station 230, or both brush stations if necessary, ofthe scrubber in FIG. 2.

[0055] Thus, embodiments of the present invention, which may includecleaning environment and methods for cleaning substrates, such as, forinstance, post copper CMP substrates, alleviate the problems of brushloading without affecting the quality of the copper and dielectriclayers. Furthermore, these embodiments, when used to clean post copperCMP substrates have the capability of removing surface and subsurfacecontaminants from the copper and dielectric layers.

[0056] Hence, methods and apparatus for cleaning semiconductorsubstrates after polishing of copper film have been described. Althoughspecific embodiments, including specific equipment, parameters, methods,and materials have been described, various modifications to thedisclosed embodiments will be apparent to one of ordinary skill in theart upon reading this disclosure. Therefore, it is to be understood thatsuch embodiments are merely illustrative of and not restrictive on thebroad invention and that this invention is not limited to the specificembodiments shown and described.

What is claimed is:
 1. A cleaning solution formed by mixing an amount of citric acid and an amount of ammonia in deionized water, the amount of citric acid being in a range from about 0.18% by weight to about 0.22% by weight, the amount of ammonia being in a range from about 0.0225% by weight to about 0.0275% by weight, and the cleaning solution having a pH of about
 4. 2. The cleaning solution of claim 1, wherein a source of ammonia is a solution formed by mixing ammonia in deionized water.
 3. The cleaning solution of claim 2, wherein the amount of citric acid is about 0.2% by weight and the amount of ammonia is about 0.025% by weight.
 4. A concentrated cleaning solution for cleaning semiconductor substrates, the concentrated cleaning solution being formed by mixing an amount of citric acid and an amount of ammonia in deionized water, the amount of citric acid being in a range from about 3.6% by weight to about 4.4% by weight, the amount of ammonia being in a range from about 0.45% by weight to about 0.55% by weight, and the concentrated cleaning solution having a pH of about 3.8.
 5. The concentrated cleaning solution of claim 4, wherein a source of ammonia is a solution formed by mixing ammonia in deionized water.
 6. The concentrated cleaning solution of claim 5, wherein the amount of citric acid is about 4% by weight and the amount of ammonia is about 0.5% by weight.
 7. A method for cleaning a semiconductor substrate having a polished copper layer comprising: providing a concentrated cleaning solution formed by mixing an amount of citric acid and an amount of ammonia in deionized water, the amount of citric acid being in a range from about 3.6% by weight to about 4.4% by weight, the amount of ammonia being in a range from about 0.45% by weight to about 0.55% by weight, and the concentrated cleaning solution having a pH of about 3.8; placing the semiconductor substrate having a polished copper layer in a scrubbing apparatus; mixing one unit volume of the concentrated cleaning solution with 20 unit volumes of deionized water proximate to the scrubbing apparatus to obtain a cleaning solution having a pH of about 4; and scrubbing the semiconductor substrate in the scrubbing apparatus in the presence of the cleaning solution. 